1. Field of the Invention
This invention relates to a process of producing a resin sheet or film from thermoplastic resins and to a molding screw therefor.
2. Description of the Prior Art
Crystalline thermoplastic resins such as polypropylene and polyethylene have been utilized in the various packaging fields and the like as sheets and containers due to their easiness in being thermoformed.
In producing the crystalline thermoplastic resin sheets as represented by polypropylene, as described above, to improve the transparency of these sheets, it has been practiced that the molten resin sheet be quenched to control the crystalline state. In a quenching process of this type, water cooling is the most efficient process. However, this process has a problem in that it is difficult to cool uniformly a molten film-shaped resin extruded from an extrusion die. The present applicant has proposed a process of using a multi-stage slit, through which cooling water flows down, as a process of uniform water-cooling the molten film-shaped resin (Japanese Patent Kokai (Laid-Open) No. 58-203018).
However, even if the cooling conditions are controlled, the cooling effect thereof is naturally limited, whereby the transparency, surface uniformity of the resin sheets as the molded items have not always been satisfactory.
As a result of various studies carried out by the inventors of the present invention, it was found that conditions of extrusion have a great influence at a stage of the molten film-shaped resin, where the molten film-shaped resin extruded has not yet come into contact with the cooling water, i.e. at a stage covering from the feeding the material to a resin extruder to the extruding of the material, and a process in which these extruding conditions are combined with a slit water cooling method, was proposed (Patent Kokai (Laid-Open) No. 62-146611 and Patent Application No. 61-202146).
Now, a construction of a screw is listed as one of important factors among the above-described extruding conditions.
One of the commonest screws is shown in FIG. 1 for example. This screw, like a full-flighted metering screw, having a feeding section 51, a compressing section 52 and a metering section 53, which are positioned successively from a side of feeding resin to a side of extruding resin, has as its main purpose the increase in a compression ratio for satisfactory kneading of molten resin. Accordingly, although the kneading of the resin can be carried out relatively well, several problems have been presented because the compression ratio is high which is accompanied with high shearing. More specifically, the pressure deviation at the screw tip is remarkable at the time of high speed extrusion, so that molded items of high quality, excellent in accuracy of thickness and the like cannot be obtained stably. Furthermore, heat is generated due to the high shear rate, whereby the resin temperature is raised to an unncessarily high temperature, thus presenting such disadvantages the deteriorated resin leads to lowered physical properties of the molded items and take-up and sizing in the extrusion molding become difficult.
Because of this, there have been proposed a screw having a construction wherein the compression ratio is made relatively small and set at 1 as the case may be to thereby control occurrence of shearing heat, another construction of a screw of two-stage extrusion, wherein a shearing section is provided at the intermediate portion of the screw, and the like. However, in either of these cases, kneading became unsatisfactory, with the result that non-uniformity in resin temperature and dispersion failure of various additives took place, whereby it was difficult to obtain satisfactory molded items and the scope of applications was extremely limited. Particularly, with the screw of the two-stage extrusion, a shearing beyond need occurs at the intermediate shearing portion, and such problems are encountered that the resin is deteriorated due to raised resin temperature and difficulties in cooling and sizing due to the extrusion of the resin heated to high temperature.
Now, in order to solve these problems, the present applicant has proposed an extruder wherein the compressing section of the screw has a relatively low compression ratio, i.e. a moderate compressing section is provided (Patent Kokai (Laid-Open) No. 61-144322). A screw wherein the above-described one is further improved is proposed (Patent Kokai (Laid-Open) Nos. 62-41015 and 62-4101687).
However, in the screw provided at the intermediate portion thereof with the shearing section, in order to control the shearing heat at this shearing section at a low temperature, it becomes necessary to make the lengths of the feeding section and the compressing section satisfactorily long, which is not preferable. Even if these sections are lengthened, kneading in the shearing section is not satisfactorily when carrying out extrusion molding at high speed. As the case may be, a large quantity of unmolten resin flows into the shearing section, whereby fluctuations in an extrusion flow rate may take place and satisfactory molded items cannot be obtained, so that the extrusion molding cannot be carried out with high productivity, at high speed and stably.